Private security guards from Blackwater Worldwide participated in some of the C.I.A.’s most sensitive activities — clandestine raids with agency officers against people suspected of being insurgents in Iraq and Afghanistan and the transporting of detainees, according to former company employees and intelligence officials.

The raids against suspects occurred on an almost nightly basis during the height of the Iraqi insurgency from 2004 to 2006, with Blackwater personnel playing central roles in what company insiders called “snatch and grab” operations, the former employees and current and former intelligence officers said.

Several former Blackwater guards said that their involvement in the operations became so routine that the lines supposedly dividing the Central Intelligence Agency, the military and Blackwater became blurred. Instead of simply providing security for C.I.A. officers, they say, Blackwater personnel at times became partners in missions to capture or kill militants in Iraq and Afghanistan, a practice that raises questions about the use of guns for hire on the battlefield.

Separately, former Blackwater employees said they helped provide security on some C.I.A. flights transporting detainees in the years after the 2001 terror attacks in the United States.

The secret missions illuminate a far deeper relationship between the spy agency and the private security company than government officials had acknowledged. Blackwater’s partnership with the C.I.A. has been enormously profitable for the North Carolina-based company, and became even closer after several top agency officials joined Blackwater.